PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD 300 PM EDT MON APR 30 2018 30-DAY OUTLOOK DISCUSSION FOR MAY 2018 THE MAY 2018 FORECAST IS UPDATED TO ACCOUNT FOR THE LATEST MODEL GUIDANCE AND SHORT TERM WEATHER PHENOMENA THAT ARE LIKELY TO EXERT A STRONG INFLUENCE ON THE MONTHLY-AVERAGED CLIMATE. OUR MID-MONTH ASSESSMENT OF LOW-FREQUENCY CLIMATE VARIABILITY IS LARGELY UNCHANGED. THE MJO IS CURRENTLY WEAK, BUT COULD REORGANIZE OVER THE WESTERN HEMISPHERE DURING WEEK-2. WHILE THIS IS NOT EXPECTED TO YIELD A STRONG EXTRATROPICAL RESPONSE, THERE IS VERY MODEST EMPIRICAL EVIDENCE THAT THIS EVOLUTION COULD FAVOR A COOLING TREND, RELATIVE TO NORMAL, OVER PARTS OF THE PLAINS DURING THE MONTH. THE TEMPERATURE FORECAST IS UPDATED TO REFLECT MAJOR WARMING TRENDS FORECAST OVER BOTH THE EASTERN AND WESTERN PARTS OF THE CONUS: THE FORMER DURING THE FIRST FIVE DAYS OF THE MONTH, THE LATTER DURING THE DAY 6-14 PERIOD. THE LATEST MODEL GUIDANCE FROM THE CFS AND OUR OFFICIAL WEEK-2 AND WEEK 3-4 FORECAST PRODUCTS ALL SUPPORT A GENERAL TILT TOWARD ABOVE-NORMAL TEMPERATURES OVER THE ENTIRE CONUS. THE ODDS OF ABOVE-NORMAL TEMPERATURES ARE THE GREATEST OVER PARTS OF THE INTERIOR WEST, WHERE A MAJOR, PROLONGED WARMUP IS FORECAST FROM ABOUT MAY 6 ONWARD. A WEAKNESS IN PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES IS EVIDENT OVER PARTS OF THE PLAINS, MISSISSIPPI VALLEY, AND SOUTHEAST. PERIODIC UPPER-LEVEL TROUGHING IS INDICATED FOR THESE REGIONS, LIMITING FORECAST CONFIDENCE. ABOVE-NORMAL TEMPERATURES ARE ALSO FORECAST FOR ALASKA, WITH THE BEST ODDS ACROSS THE NORTH SLOPE, WHERE LONG-TERM WARMING TRENDS ARE DOMINANT. THE PRECIPITATION FORECAST FIRST TAKES INTO ACCOUNT AREAS OF HEAVY RAINFALL FORECAST AT THE BEGINNING OF THE MONTH. THE HEAVIEST RAIN RELATIVE TO NORMAL IS FORECAST OVER TEXAS, WITH A SECONDARY REGION OVER THE CENTRAL PLAINS AND UPPER MIDWEST. THE MONTHLY-AVERAGED CIRCULATION OVER NORTH AMERICA IS EXPECTED TO FAVOR ENHANCED SOUTHERLY FLOW ACROSS MUCH OF THE CENTRAL CONUS, SO PRECIPITATION EVENTS ARE LIKELY TO OCCUR THROUGH THE MONTH. THE FIRST WEEK OF MAY IS FORECAST TO BE QUITE DRY OVER PARTS OF THE TENNESSEE VALLEY, SOUTHERN APPALACHIANS, AND THE SOUTHEAST. BEYOND WEEK-1, PRECIPITATION SIGNALS ARE WEAK OVER MUCH OF THE EASTERN CONUS, LEADING TO EXPANDED COVERAGE OF AN EQUAL CHANCES FORECAST FOR PARTS OF THE EASTERN SEABOARD. UPPER-LEVEL RIDGING IS FORECAST OVER THE NORTHWEST, LEADING TO A FORECAST FAVORING BELOW-AVERAGE PRECIPITATION. ANOMALOUS SOUTHERLY FLOW IS FORECAST OVER ALASKA, ESPECIALLY EARLY IN THE MONTH; THIS FAVORS ABOVE-NORMAL PRECIPITATION AT REASONABLY HIGH PROBABILITIES FOR THE SOUTHWESTERN PARTS OF THE STATE. THE ORIGINAL MAY 2018 DISCUSSION FOLLOWS BELOW: --------------------------------------------------------------- THE MAY 2018 IS ISSUED AGAINST THE BACKDROP OF A TRANSITIONING ENSO STATE (TO ENSO-NEUTRAL) AND FOLLOWING A PROLONGED STRETCH (INCLUDING FORECASTS THAT EXTEND THROUGH THE END OF APRIL) OF BELOW-NORMAL TEMPERATURES ACROSS MANY PARTS OF THE CONUS. AS WITH ANY CLIMATE FORECAST, WE MUST BE COGNIZANT OF LONG-TERM TRENDS; BOTH A 15-YEAR OCN AND A LEAST-SQUARES LINEAR FIT REVEAL STRONG WARMING TRENDS ACROSS THE SOUTHERN AND EASTERN CONUS DURING LATE SPRING AND EARLY SUMMER. SLIGHTLY NEGATIVE TEMPERATURE TRENDS ARE OBSERVED FOR A SMALL PORTION OF THE NORTHERN HIGH PLAINS AND NORTHERN ROCKIES. PRECIPITATION TRENDS ARE NOT NEGLIGIBLE THIS TIME OF YEAR EITHER; AN INCREASING (DECREASING) TREND IN LATE SPRING AND EARLY SUMMER PRECIPITATION HAS BEEN OBSERVED ACROSS MUCH OF THE NORTHERN AND EASTERN CONUS (SOUTHWESTERN CONUS). THESE LONG-TERM TRENDS FORM THE STARTING POINT FOR THE MAY 2018 OUTLOOK, AT WHICH POINT WE CONSIDER SUBSEASONAL TO SEASONAL CLIMATE VARIABILITY, INCLUDING ENSO, SOIL MOISTURE, AND VARIOUS STATE-OF-THE-ART DYNAMICAL MODEL FORECASTS. THE TEMPERATURE OUTLOOK IS CONSTRUCTED WITH THE AFOREMENTIONED TRENDS IN MIND, BUT TWEAKED COLDER ACROSS PARTS OF THE EAST-CENTRAL CONUS CENTERED ON THE MISSISSIPPI RIVER. THIS IS DUE TO VERY HIGH SOIL MOISTURE CONTENT AND THE LATEST MODEL GUIDANCE FROM THE NMME AND ECMWF. OVER THE MID-ATLANTIC AND NORTHEAST THE OBJECTIVE GUIDANCE, TAKEN IN ITS ENTIRETY, STILL EXHIBITS A SLIGHT TILT TOWARD ABOVE-NORMAL TEMPERATURES DESPITE DYNAMICAL MODEL WEEK 3-4 GUIDANCE, FOR EXAMPLE, THAT TILTS COLD THROUGH THE FIRST HALF OF MAY. PROBABILITIES FAVORING ABOVE-NORMAL TEMPERATURES ARE HIGHEST OVER THE SOUTHWEST, WHERE LONG-TERM TRENDS, NEGATIVE SOIL MOISTURE ANOMALIES, LAGGED ENSO REGRESSIONS, AND DYNAMICAL MODEL GUIDANCE ARE ALL IN AGREEMENT. OVER THE NORTHERN HIGH PLAINS, A SIMILAR ALIGNMENT OF GUIDANCE POINTS MORE TOWARD BELOW-NORMAL TEMPERATURES. THERE IS BROAD CONSENSUS FAVORING ABOVE-NORMAL TEMPERATURES FOR ALASKA, WITH THE HIGHEST PROBABILITIES NEAR THE NORTHERN AND WESTERN COASTS, WHERE ICE BREAKUP IS LIKELY TO OCCUR EARLIER THAN NORMAL. THE PRECIPITATION OUTLOOK IS CONSTRUCTED IN SIMILAR FASHION TO THE TEMPERATURE OUTLOOK, STARTING WITH LONG-TERM TRENDS AND ADJUSTING FOR SUBSEASONAL TO SEASONAL VARIABILITY MANIFEST IN EITHER STATISTICAL OR DYNAMICAL GUIDANCE. LAGGED ENSO REGRESSIONS AND THE PREPONDERANCE OF DYNAMICAL GUIDANCE CONSTRUCTIVELY INTERFERE WITH LONG-TERM TRENDS OVER MUCH OF THE WEST, SO BELOW-NORMAL PRECIPITATION IS MODESTLY FAVORED FOR PARTS OF THE WEST EXTENDING FROM THE NORTHWEST COAST SOUTHEASTWARD TO PARTS OF THE SOUTHERN HIGH PLAINS. ABOVE-NORMAL PRECIPITATION IS FAVORED SLIGHTLY OVER THE NORTHERN HIGH PLAINS WHERE LONG-TERM TRENDS ARE FAIRLY STRONG, AND CALIBRATED GUIDANCE FROM THE NMME HAS A COHERENT WET SIGNAL. OVER THE EASTERN CONUS THERE ARE SOME CONFLICTING SIGNALS, BUT THE MAJORITY OF FORECAST GUIDANCE, INCLUDING THE NMME, LONG-TERM TRENDS, AND THE ECWMF, TILTS SLIGHTLY TOWARD ABOVE-NORMAL PRECIPITATION. THE VARIOUS TOOLS ARE IN QUITE GOOD AGREEMENT OVER ALASKA, BROADLY FAVORING ABOVE-NORMAL PRECIPITATION, WITH THE BEST ODDS IN SOUTHWESTERN ALASKA. FORECASTER: STEPHEN BAXTER THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1981 AND 2010, FOLLOWING THE WORLD METEOROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3 COMPLETE DECADES AS THE CLIMATE REFERENCE PERIOD. THE PROBABILITY ANOMALIES FOR TEMPERATURE AND PRECIPITATION BASED ON THESE NEW NORMALS BETTER REPRESENT SHORTER TERM CLIMATIC ANOMALIES THAN THE FORECASTS BASED ON OLDER NORMALS. THE NEXT MONTHLY OUTLOOK...FOR JUN ... WILL BE ISSUED ON THU MAY 17 2018 THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1981-2010 BASE PERIOD. $$